mlxsw: spectrum_router: Propagate neighbour update errors

mlxsw: spectrum_router: Propagate neighbour update errors

Next patch will add offload indication to neighbours, but the indication
should only be altered in case the neighbour was successfully added to /
deleted from the device.

Propagate neighbour update errors, so that they could be taken into
account by the next patch.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>